Two-strand yarns may be produced by spinning or twisting together two strands in which the fibre tails have been wrapped by an air-jet (eg Plyfil) or in which the alternating strand twist is trapped during the operation (eg Sirospun). Such yarns have enhanced strength and abrasion resistance relative to singles yarns but in worsted processing have an average cross-section of around 80 or more fibres. It would be very useful to produce a weavable singles yarn of a structure which may be of significantly smaller cross-section, with say around 50-60 fibres or less. However, singles yarns of such size to date have tended to have inadequate strength and abrasion resistance for weaving and knitting applications.
It was recognized by Peirce [Peirce, F. T.; Textile Research Journal, 1947, 17, p123], Morton and Yen [Morton, W. E. and Yen, K. C. J.; Journal of the Textile Institute, 1952, 22, T.463], and Morton [Morton, W. E.; Annales Scientifiques Textiles Belges, 1956, p29], that fibre migration, or entanglement, must occur during twist insertion to give the resulting yarn strength and abrasion resistance. In relation to the fibre strand emerging from the front roller nip, Morton stated in part that " . . . since the length of the fibre path increases from the core to the surface, so also must the tension in the fibres. At any given instant, those forming the outer layer of the yarn follow the longest path and are conseqently highly stressed; and furthermore the curvature of their path is also the largest" It has been shown by the above authors that these highly stressed fibres will tend to migrate toward the axis of the yarn in order to achieve a lower tension condition. However, " . . . as soon as the trailing end of the fibre emerges from the nip of the front rollers, tension in the fibre must drop to zero. It is then in no condition to do other than suffer expulsion to the surface, where it will appear as (a) projecting fibre." In his concluding remarks, Morton states, "A further practical outcome is that, since wild, or wildish fibres, (we must recognise that there are degrees of wildness) are unlikely to contribute their fair share to the strength of the yarn, the width of the ribbon of drawn-out roving should be limited as much as possible".
International patent publication WO94/01604 (PCT/NZ93/00055) by Wool Research Organization of New Zealand discloses a number of practical techniques for applying the above concepts to a single drafted assembly or strand of fibres as the strand is spun from a drafting system. In one of these techniques, a guide oscillates the strand laterally so as to cyclically vary the tension in the fibres of the strand. By varying the tension in this way, the fibres are caused to migrate cyclically between the core and the surface of the resultant yarn. In another arrangement, the drafted strand is passed through an additional pair of nip rollers located immediately downstream of the front drafting rollers. The nip rollers are driven at a lower speed than the delivery speed of the front drafting rollers, a negative draft which induces an "overfeed" zone in which the fibres are found to randomly alter their positions at the nip. There is thus a random migration of the fibres between the core and the surface of the yarn. In a third arrangement, the drafted strand is allowed to spread sufficiently laterally for "sub-groupings" to form in which the fibres are false twisted to form separate sub-strands that are then twisted together in a recombined yarn.
The proposal in WO94/01604 for guide oscillation has some similarities to various proposals for forming two-strand yarns from a pair of separate strands, disclosed or discussed eg in U.S. Pat. No. 3,599,416, in Australian patents 438072 and 473153, and in D. Plate et al, J. Text. Inst. 73 (No. 3, 1982), p. 99, and 74 (No. 6, 1983), p. 320. This class of two-strand spinning processes embraces, inter alia, the present applicant's technology known as the "Sirospun" process. The possible existence of pre-twisting of small fibre sub-groupings in the twist triangle of two-strand spinning systems is discussed in Neckar et al, Melliand Textilberichte [English edition], August, 1985, p. 605.
Harakawa et al (J. Text. Machinery Soc. Japan, 43 (No. 11, 1990), T98 and 41 (1988), T(177) propose a device in which the strand emerging from the front rollers is drawn down to a hollow spindle which can be oscillated laterally. The yarns so produced have different fibres on the outside according to the side from which they emerged and the position of the hollow spindle. A corresponding disclosure is to be found in Japanese patent publication 57-029615.
U.S. Pat. No. 4,418,523 disclosed a notched roller for providing fancy yarns in spinning-twisting machines, where the core is false-twisted and wrapped with a filament.